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Can Using the Wrong Octane Fuel Lead to Disaster?

By Paul Stenquist March 30, 2010 7:30 amMarch 30, 2010 7:30 am

Last week, Gunnar Heinrich, who runs the site Automobiles De Luxe, wrote that a 2010 Cadillac SRX Turbo he had borrowed from General Motors for a review sustained serious internal damage during a test drive and that he had to be plucked from the side of a mountain road.

According to Mr. Heinrich’s article, using the wrong fuel was partly to blame:

The V-6 in the Cadillac requires a minimum 91 octane. Lab tests had shown that the SRX was running on regular, which meant that yours had inadvertently fed the car 88 octane gas and not the factory mandated premium.

At some point during travel, between 2,000-2,500 r.p.m. — or normal highway cruising speed — the engine’s management system had adjusted the air fuel mixture to work too lean causing a retarded spark – but crucially – it allowed for a simultaneous turbo boost, which led to a catastrophic pressure build-up in the cylinder chambers.

Could a difference of only three octane cause a vehicle to break down? Asked about the incident, a Cadillac spokesman, David Caldwell, said in an interview that the low-octane fuel that Mr. Heinrich bought was a contributing factor but that it wasn’t the principal cause. So the quick and simple answer is no. Using regular fuel in a turbocharged engine or any engine designed for premium generally won’t lead to immediate and serious damage. The long answer is more complicated.

The demon that led to the demise of the 2.8-liter turbocharged V-6 in this Cadillac was detonation, Mr. Caldwell said. It’s a condition caused by the fuel igniting prematurely and in more than one location within the combustion chamber. The multiple flame fronts cause extremely high cylinder pressure that can pound engine parts to death. High-octane fuels don’t ignite as readily as lower-octane blends, so detonation is less likely to occur when they’re used.

But nearly every engine produced today is equipped with a knock sensor. On engines designed for premium fuel, this device compensates when regular is used. Mounted on the engine, the sensor detects the rattle of detonation and sends a signal to the engine computer, which changes the calibration to eliminate the cause. In most cases, that means retarding ignition timing. With the spark coming later, less heat is generated, so unburned fuel isn’t ignited prematurely and detonation doesn’t occur.

In Mr. Heinrich’s case, a failure might have rendered the computer incapable of responding to conditions. Mr. Caldwell said G.M. was still testing and could not say conclusively what caused the problem. G.M. engineers are checking to make sure the calibration can cope with even the most extreme circumstances.

Extreme circumstances were certainly part of the equation in Mr. Heinrich’s incident. According to his article, he was driving up a mountain near Ventura, Calif., with the engine at about 2,000 r.p.m. That’s a high-load situation, which in itself causes high cylinder pressure and combustion chamber heat. Coupled with the detonation-prone nature of regular fuel, those conditions put the knock sensor into overdrive. As the spark was retarded, the engine calibration might have allowed for more boost to avoid a significant loss of power – either by design or because of component failure. That diminished the effectiveness of late spark in regard to eliminating detonation. Consequently, the hammering in the combustion chambers became so violent that a connecting rod was damaged.

Second, power-plant engineering is a tricky business. In high-performance engines, power versus reliability is a balancing act, and a slight hiccup in an engine calibration can tip the scales. But no manufacturer wants to be saddled with failures, and you can be sure that it doesn’t take more than one very major and public incident to send the engineers racing back to their calibration computers.

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